The heat of combustion for wood is expressed as the high heating value (HHV) or low heating value (LHV). The HHV can be thought of as the gross amount of energy trapped in an odMg of wood while the LHV is the net energy after accounting for the moisture content of the wood. The HHV can be considered a theoretical value while the LHV is closer to the utilizable energy.

The quantity of usable heat produced by the complete combustion of pine logs (LHV) will vary depending upon the moisture content [4]. Although a cubic meter of wood contains the same mass of cellulose either green, air dried or oven dried, the amount of usable heat produced is greater for oven-dried wood (Table 10.1). This is because energy is required to turn water into steam. The more water the wood contains, the more energy will be used to produce steam.

Two methods are used when calculating the moisture content. In some reports, the denominator includes both wood and water and in other reports it does not include water (i. e. oven-dry basis). When the method is not specified, this difference can cause confusion. For example, if someone says wood has a moisture content of 50%, it would not be clear if this means half of the mass is water (i. e. green basis) or if 33% of the mass is water (i. e. 1/3 water and 2/3 wood). Therefore, in this chapter, we will include the designation “od” to indicate the denominator does not include water.

The energy contained in pine logs will also vary depending upon the resin content. The heat of combustion for pine increases by about 16.8 kWh/dry tonne for each percentage point increase in extractive content. Pine logs with high resin might have 20% more energy than logs with no resin [5]. In fact, the heat of combustion of pine resin can be higher than coal (Table 10.1). Certain “hard pines” produce more resin than other species. Pines known for their ability to produce lots of resin include Pinus elliottii and Pinus palustris. Wood from pines from the southern United States may contain about 5% extractives, but heartwood segments from old growth may contain 30-35% extractive content. This will cause the wood to be dense enough to sink when placed in water. This type of wood is commonly referred to as “fatwood” or “lighter wood” and is sold as kindling on the Internet for about $2.50 per kg (or $2500 per tonne). Unfortunately, the demand is generally low and, therefore, it may take some time to sell one Mg of “fatwood.”